html {
  font-size: 10px;
}

.foo {
  font-size: calc(1.6rem + 3.1999999999999997 * ((100vw - 42rem) / 86));
  --font-size: calc(1.6rem + 3.1999999999999997 * ((100vw - 42rem) / 86));
}

@media screen and (min-width: 1280px) {

  .foo {
    font-size: 4.8rem;
    --font-size: 4.8rem;
  }
}

@media screen and (max-width: 420px) {

  .foo {
    font-size: 1.6rem;
    --font-size: 1.6rem;
  }
}